Contemporary electronic circuitry often includes resistor components produced in the form of thin layers or films of resistive materials supported on an insulating substrate. Individual resistive elements of this type can be very small in size. Their manufacture, however, is generally done in the form of large repetitive two-dimensional arrays using wafer-scale processing techniques.
There are two distinct versions of film resistors. One version, referred to as thin film, is made using vacuum-deposited films of materials a few hundred to a few thousand angstroms in thickness. Different resistor values are achieved by forming these thin films into patterns having different length-to-width ratios, known as the aspect ratio. The patterning is done by photolithography. A layer of photosensitive polymer is deposited onto the resistive thin film. It is then exposed to a light source directed through a photo mask carrying an image of the desired pattern. The light changes the polymer solubility by special solvents, thereby exposing the underlying resistive film in selected regions. The exposed resistive film is then removed using aggressive chemical agents or plasma etching.
The range of resistivity values available by changing the film thickness is limited, to a factor of about ten, ranging from about 50 to 500 ohms per square (the unit of which is the resistance across a pattern having an aspect ratio of one). Different values are, therefore, achieved primarily by varying the aspect ratio geometrically using photolithography. Aspect ratios can be varied by a factor of many thousands, depending upon the minimum line width achievable.
Another common resistor manufacturing method is referred to as thick film. In this technology, a relatively limited range of geometrical patterns is employed and the primary way of varying the resistance value is achieved by changing the specific resistivity of the film material itself. These materials are a mixture of fine powders of glass and powders of resistive substance, typically ruthenium dioxide or ruthenate compounds. These powders are mixed with an organic vehicle to form viscous ink. The ink is deposited onto an insulating substrate in patterns using stencil screen-printing techniques. After the volatile vehicle is removed in a low temperature oven, the system is raised to a higher temperature to fuse the glass constituent, resulting in a partially conducting glaze. The specific resistivity of the glaze depends upon the relative proportions of glass and conductive phase; the resistivity can range over a factor of ten or eleven decades (i.e. orders of magnitude). The thickness of the final film is about a half a mil.
For a given resistance value, resistive elements made from thin film are generally of higher quality than those of thick film. They have a smaller temperature coefficient of resistance (TCR), a lower current noise, a smaller voltage coefficient of resistance (VCR), and greater stability throughout service life. However, for a given size, thin film elements are limited in maximum value achievable compared with thick film elements. By using thick film compositions, with reduced resistive phase, resistors can be made with a resistance several orders of magnitude higher than for thin film.
The ohmic value of thick film resistors varies significantly with its composition. At very low ohmic values, the proportion of glass phase is very low, making it unstable. As a result, it is difficult to form a continuous matrix of glass and thereby completely isolate the conductive phase of the resistor from attack by atmospheric agents. In compositions with middle level ohmic values, stability is improved. As the proportion of resistive phase is further reduced to reach higher values, however, the quality degrades in relation to the degree of dilution of the resistive phase. The TCR is higher, the current noise is greater, the current-voltage linearity is poorer, and the service life is reduced.
One way of counteracting this tendency is to use thick film compositions of moderate specific resistivity and rely on ways of achieving higher aspect ratios. As with thin films, the maximum aspect ratio achievable, within a given area, is a direct function of the minimum line width obtainable (i.e. it is inversely proportional to the square of the line width). The finer the line width, the higher the aspect ratio.
With conventional stencil screen techniques the minimum line width attainable is on the order of 10 mils. This would allow an element of 10×40 mils to be made by screen-printing with an aspect ratio of four.
An earlier idea, disclosed in U.S. Pat. No. 5,521,576 to Collins, is the discovery that higher aspect ratio patterns of thick film inks could be achieved using extrusion of ink through a small orifice in a pen tip, much as in writing. By this method, it was demonstrated that patterns based on line widths as narrow as 5.5 mils could be deposited directly onto substrates resulting in aspect ratios nearly an order of magnitude greater than could be produced by screen-printing. This means that in order to achieve a given resistance value, in a given size, an ink composition having nearly an order of magnitude lower resistivity could be used with a concomitant improved performance. Despite this improvement, the ability to write resistive lines with even finer widths is needed.
The present invention is directed to satisfying this objective.